Electric linear-motion actuator and electric brake assembly

ABSTRACT

An actuator has planetary rollers disposed between a rotor shaft of an electric motor and an outer ring member fixed around the rotor shaft. The planetary rollers are rotated about the axis of the rotor shaft and about their own axes, thereby converting rotary motion of the rotor shaft to linear motion of the planetary rollers A helical groove is formed in the radially outer surface of each planetary roller in which a helical rib formed on the radially inner surface of the outer ring member is received. The helical groove has a pitch equal to that of the helical rib and a lead angle different from that of the helical rib. The amount of the linear motion of the planetary rollers relative to the amount of the rotary motion of the rotor shaft is determined by the difference in lead angle between the helical groove and the helical rib.

This application is a continuation application of Ser. No. 11/989,231(now U.S. Pat. No. 8,109,370, issued Feb. 7, 2012 filed Mar. 19, 2008),which is the U.S. National Stage of International ApplicationPCT/JP2006/314839, filed Jul. 27, 2006, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to an electric linear-motion actuator forconverting the rotary motion of an electric motor to a linear motion,thereby linearly driving a member to be driven, and an electric brakeassembly using the electric linear-motion actuator to press a brakemember against a member to be braked.

BACKGROUND ART

Many electric linear-motion actuators for converting the rotary motionof an electric motor to a linear motion, thereby linearly driving amember to be driven, include a ball-screw mechanism or a ball-rampmechanism as their motion converter mechanism. Also, in order to obtainlarge linear driving force with a small-capacity electric motor, many ofsuch actuators include a speed reducing mechanism such as a planetarygear speed reducing mechanism (see e.g. JP patent publication 6-327190A(FIGS. 1 and 5)).

On the other hand, many of the existing vehicle brake assemblies arehydraulic ones. But with the recent introduction of sophisticated brakecontrol systems such as anti-lock brake systems (ABS), electric brakeassemblies are gathering attention because they can perform suchsophisticated control without the need for complicated hydrauliccircuits and they can be designed compactly. Such electric brakeassemblies include an electric motor which is actuated in response toe.g. signals indicating that the brake pedal is depressed, and anelectric linear-motion actuator as described above which is mounted in acaliper body for pressing a brake member against the member to be brakedwhen the motor is actuated (see e.g. JP patent publication 2003-343620A(FIG. 1)).

Ordinarily, electric brake assemblies are mounted on a vehicle eachunder one of the springs of the vehicle, and thus, it is desired thatsuch brake assemblies operate stably under the influence of vibrationstransmitted from the ground, and can be designed compactly.

SUMMARY OF THE INVENTION

Ball-screw mechanisms and ball-ramp mechanisms used in such conventionalelectric linear-motion actuators have the ability to increase power tosome extent by motion converting means that moves along a thread havinga lead or an inclined cam surface, but cannot increase power to a levelrequired e.g. in electric brake systems. That is, while power can beincreased by reducing the lead angle of the thread or the inclinationangle of the cam surface, in the case of ball-screw mechanisms, if thelead angle of the thread is reduced, the ball diameter decreases, sothat the load capacity decreases. In the case of ball-ramp mechanisms,if the inclination angle of the cam surface is reduced, it is difficultto ensure a sufficient stroke of the linear motion.

Thus, with electric linear-motion actuators using such motion convertingmeans, a separate speed reducing mechanism as described above is mountedto increase the driving force. But if a separate speed reducingmechanism such as a planetary gear speed reducing mechanism is mounted,it becomes difficult to compactly design the electric linear-motionactuator.

In order to avoid this problem, the present applicant has proposed, asan electric linear-motion actuator capable of sufficiently increasingpower without mounting a separate speed reducing mechanism, a mechanismincluding planetary rollers disposed between the radially outer surfaceof the rotor shaft of an electric motor and the radially inner surfaceof an outer ring member fixed in position around the radially outersurface of the rotor shaft such that when the rotor shaft rotates, theplanetary rollers rotate about the axis of the rotor shaft whilesimultaneously rotating about their own axes. A helical rib is formed onthe radially outer surface of the rotor shaft or the radially innersurface of the outer ring member, while circumferential grooves areformed in the radially outer surface of each planetary roller at pitchesequal to the pitches of the helical rib. The helical rib is engaged inthe circumferential grooves of the respective planetary rollers so thatwhen the planetary rollers rotate about the axis of the rotor shaftwhile simultaneously rotating about their own axes, the planetaryrollers also move in the axial direction of the rotor shaft relative tothe rotor shaft. Thus, this mechanism can convert the rotary motion ofthe rotor shaft to the linear motion of the planetary rollers (JP patentapplication 2005-6714).

With this arrangement, because circumferential grooves are formed in theradially outer surface of each planetary roller for engaging the helicalrib, the amount of the linear motion of the planetary rollers relativeto amount of the rotary motion of the rotor shaft, i.e. the reductionrate of the linear motion is determined by the lead angle of the helicalrib only. Thus, by reducing the lead angle, it is possible tocorrespondingly increase the reduction rate of the linear motion andthus the linear driving force. But there is a limit below which the leadangle of the helical rib cannot be reduced. Thus, there is a limit abovewhich the linear driving cannot be increased.

An object of the present invention is therefore to increase the lineardriving force in a linear-motion actuator of the type in which therotary motion of the rotor shaft of an electric motor is converted tothe linear motion of planetary rollers disposed between the rotor shaftand an outer ring member by rotating the planetary rollers about theaxis of the rotor shaft and also about their own axes.

In order to achieve this object, the present invention provides anelectric linear-motion actuator for linearly driving a member to bedriven by converting the rotary motion of an electric motor to a linearmotion, characterized in that a plurality of planetary rollers aredisposed between a radially outer surface of a rotor shaft of theelectric motor and a radially inner surface of an outer ring memberfixed in position around the radially outer surface of the rotor shaftsuch that when the rotor shaft rotates, the planetary rollers rotateabout the axis of the rotor shaft while simultaneously rotating abouttheir own axes, that a helical rib is formed on one of the radiallyouter surface of the rotor shaft and the radially inner surface of theouter ring member, and that a helical groove is formed in a radiallyouter surface of each of the planetary rollers, the helical rib being inengagement with the helical grooves of the respective planetary rollers,the helical grooves being equal in pitch to the helical rib, anddifferent in lead angle from the helical rib, whereby when the planetaryrollers rotate about the rotor shaft while simultaneously rotating abouttheir own axes, the planetary rollers move in an axial direction of therotor shaft relative to the rotor shaft, thereby converting the rotarymotion of the rotor shaft to a linear motion of the planetary rollers.

By forming the helical groove in the radially outer surface of eachplanetary roller which is equal in pitch to the helical rib anddifferent in lead angle from the helical rib, and in which the helicalrib is engaged, the linear movement of the planetary rollers relative tothe rotary motion of the rotor shaft is determined by the difference inlead angle between the helical grooves and the helical rib. This makesit possible to increase the reduction rate of the linear motion and thusthe linear driving force.

By providing a plurality of the helical ribs, and/or a plurality of thehelical grooves in each planetary roller, it is possible to more freelydetermine the difference in lead angle between the helical rib or ribsand the helical groove or grooves.

The at least one helical rib may comprise a rib-forming member receivedin and fixed to a helical groove formed in one of the radially outersurface of the rotor shaft and the radially inner surface of the outerring member. Such a helical rib can be easily formed.

By providing means for restricting end surfaces of the rib-formingmember, which is fixed to the helical groove, it is possible to pre ventseparation of the rib-forming member, thereby making it possible to formthe helical rib, which is configured to engage in the helical groovesformed in the planetary rollers, to design dimensions.

Means for allowing the rotor shaft of the electric motor to be manuallyrotated may be provided so that if the electric motor fails, the linearpressing force of the planetary rollers can be released by manuallyturning the rotor shaft.

The present invention also provides an electric brake assembly includingan electric linear-motion actuator for converting the rotary motion ofan electric motor to a linear motion, thereby linearly driving a brakemember, and pressing the brake member against a member to be braked,wherein the electric linear-motion actuator is the above-describedelectric linear-motion actuator. With this arrangement, the brakes canbe applied with a larger linear driving force.

With the electric linear-motion actuator according to the presentinvention, by forming the helical groove in the radially outer surfaceof each planetary roller which is equal in pitch to the helical rib anddifferent in lead angle from the helical rib, and in which the helicalrib is engaged, the amount of the linear motion of the planetary rollersrelative to the amount of the rotary motion of the rotor shaft isdetermined by the difference in lead angle between the helical groovesand the helical rib. Thus, it is possible to increase the linear drivingforce.

By providing a plurality of the helical ribs, and/or a plurality of thehelical grooves in each planetary roller, it is possible to more freelydetermine the difference in lead angle between the helical rib or ribsand the helical groove or grooves.

The at least one helical rib may comprise a rib-forming member receivedin and fixed to a helical groove formed in one of the radially outersurface of the rotor shaft and the radially inner surface of the outerring member. Such a helical rib can be easily formed.

By providing means for restricting end surfaces of the rib-formingmember, which is fixed to the helical groove, it is possible to preventseparation of the rib-forming member, thereby making it possible to formthe helical rib, which is configured to engage in the helical groovesformed in the planetary rollers, to design dimensions.

Means for allowing the rotor shaft of the electric motor to be manuallyrotated may be provided so that if the electric motor fails, the linearpressing force of the planetary rollers can be released by manuallyturning the rotor shaft.

Since the above-described electric linear-motion actuator is used as theelectric linear-motion actuator in the electric brake assembly accordingto the present invention, the brakes can be applied with a larger lineardriving force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of an electric linear-motionactuator according to a first embodiment.

FIG. 2 is a sectional view taken along line II-II of FIG. 1.

FIGS. 3 a and 3 b are front views of an outer ring member and aplanetary roller, showing their helical rib and helical groove,respectively.

FIG. 4 is an enlarged sectional view of portions of the outer ringmember and the planetary roller where their helical rib and helicalgroove are in threaded engagement with each other.

FIG. 5 is a vertical sectional view of an electric brake assembly inwhich the electric linear-motion actuator of FIG. 1 is used.

FIG. 6 is a vertical sectional view of an electric linear-motionactuator according to a second embodiment.

FIG. 7 is a perspective view of a stopper of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the invention are now described with reference to thedrawings. FIGS. 1 to 4 show the first embodiment. As shown in FIGS. 1and 2, the electric linear-motion actuator of this embodiment includesan electric motor 2 mounted in a cylindrical casing 1 at one endthereof, and an outer ring member 3 mounted in the casing 1 at the otherend. Between the radially inner surface of the outer ring member 3 andthe radially outer surface of a rotor shaft 2 a of the electric motor 2,four planetary rollers 4 are disposed with a negative gap so that theplanetary rollers 4 rotate about the axis of the shaft 2 a whilesimultaneously rotating about their own axes. The rotor shaft 2 aprotrudes from the end of the casing 1 opposite to its end where theplanetary rollers 4 are mounted. At the protruding end thereof, theshaft 2 a has a hexagonal head 2 b so that the shaft 2 a can be rotatedwith e.g. a wrench. Hardening treatment is applied to the radially outersurfaces of the planetary rollers 4, and the radially inner surface ofthe outer ring member 3 and the radially outer surface of the rotorshaft 2 a with which the planetary rollers 4 are in rolling contact, forimproved wear resistance. These surfaces, which are in rolling contactwith each other, are lubricated with grease.

As shown in FIG. 3( a), two helical grooves 5 are formed in the portionof the radially inner surface of the outer ring member 3 with which theplanetary rollers 4 are in rolling contact. Rib-forming members 6 havinga square section are each received in and fixed to each of the helicalgrooves 5 to form two helical ribs on the radially inner surface of theouter ring member 3. As shown in FIG. 3( b), in the radially outersurface of each planetary roller 4, a single helical groove 7 is formedwhich is equal in pitch to the pitch between the two helical ribs anddifferent in lead angle from the two helical ribs. By providing the twohelical ribs on the outer ring member 3, it is possible to more freelydetermine the difference in lead angle of the helical groove 7 of eachplanetary roller 4 from the helical ribs. When comparing FIG. 3( a) with3(b), it appears that the helical ribs and the helical groove 7 extendin opposite directions to each other. But actually, they extend in thesame direction because the helical ribs are threadedly engaged in thehelical groove at the backside of its portion shown in FIG. 3( b).

As shown in FIG. 4, the helical groove 7 has a trapezoidal cross-sectionso that the helical ribs, which have a different lead angle from thehelical groove 7 and which are formed of the rib-forming members 6having a square cross-section, can be smoothly engaged in the helicalgroove 7. Thus, due to the difference in lead angle between the helicalribs and the helical grooves 7 of the planetary rollers 4, when theplanetary rollers 4 rotate about the axis of the shaft 2 a whilesimultaneously rotating about their own axes with their helical grooves7 in threaded engagement, with the helical ribs of the outer ring member3, the planetary rollers 4 linearly move in the axial direction.

As shown in FIGS. 1 and 2, the planetary rollers 4 are each rotatablymounted through a needle bearing 9 on one of support, shafts 8 a of acarrier 8 fitted on the rotor shaft 2 a. The planetary rollers 4 arealso supported by the carrier 8 through thrust ball bearings 10 so as tobe rotatable about their axes relative to the carrier 8. The carrier 8,which rotates about the axis of the shaft 2 a together with theplanetary rollers 4, supports a linear drive member 11 through a thrustball bearing 12. Thus, the linear motion of the planetary rollers 4 istransmitted to the linear drive member 11 through the carrier 8. Theinterior of the actuator is sealed by a boot 13 mounted between theradially outer surface of the linear drive member 11 and the outer ringmember 3, and by a film seal 14 fitted in the radially inner surface ofthe linear drive member 11, through which the rotor shaft 2 a extends.

FIG. 5 shows an electric brake assembly in which the above-describedelectric linear-motion actuator is used. The electric brake assemblyshown is a disc brake including a caliper body 21, a disc rotor 22, i.e.a member to be braked, and brake pads 23 provided in the caliper body 21and each facing one side of the rotor 22. The casing 1 of the electriclinear-motion actuator is fixed to the caliper body 21. The linear drivemember 11 presses one of the brake pads 23 against the disc rotor 22.The linear drive member 11 is rotationally fixed to the one of the brakepads 23 by means of a key. With this electric brake assembly, if theelectric motor 2 fails, it is possible to release the braking force byengaging the hexagonal head 2 b of the rotor shaft 2 a with e.g. awrench and manually turning the shaft 2 a.

FIG. 6 shows the second embodiment. The electric linear-motion actuatorof this embodiment is basically of the same structure as the firstembodiment, and differs in that a single helical groove 5 is formed inthe radially inner surface of the outer ring member 3 in which arib-forming member 6 having a square cross-section is fixedly receivedto form a single helical rib on the radially inner surface of the outerring member 3, and that the end surfaces of the rib-forming member 6 arerestricted by stoppers 15 in threaded engagement with the radially innersurface of the outer ring member 3, respectively. To prevent looseningof the stoppers 15, the stopper 15 restricting one of the end surfacesof the rib-forming member 6 nearer to the electric motor 2 has its backpressed against a shoulder 1 a of the casing 1, while the other stopper15 has its back pressed by a spring member 16. By restricting the endsurfaces of the rib-forming member 6, the stoppers 15 prevent separationof the rib-forming member 6 from the surface of the helical groove 5.

As shown in FIG. 7, the stoppers 15 are ring-shaped members each havingon its radially outer surface an external thread 15 a that threadedlyengages the radially inner surface of the outer ring member 3, and onone side thereof a shoulder 15 b that abuts one of the end surfaces ofthe rib-forming member 6. Each of the stoppers 15 is further formed withcutouts 15 c in its radially inner surface in which a tightening tool isengageable.

In the embodiments, one or two helical ribs are formed on the outer ringmember, while a single helical groove is formed in each planetaryroller. But the numbers of helical ribs and helical grooves can befreely determined depending on the desired difference in lead angletherebetween.

The electric linear-motion actuator according to this invention can beused in devices other than electric brake assemblies, too.

What is claimed is:
 1. An electric brake assembly including an electriclinear motion actuator for converting the rotary motion of an electricmotor to a linear motion, thereby linearly driving a brake member, andpressing the brake member against a member to be braked, the electriclinear-motion actuator comprising: a rotary shaft having an axis andconfigured to be rotated by the electric motor; an outer ring membermounted around a radially outer surface of the rotary shaft; a planetaryroller carrier rotatable about the axis of the rotary shaft; and aplurality of planetary rollers supported by said carrier for rotationindividually about planetary roller axes and for rotation togetheraround the axis of the rotary shaft, said planetary rollers beingdisposed between a radially outer surface of the rotary shaft and aradially inner surface of the outer ring member such that, when therotary shaft rotates, the planetary rollers rotate about the axis of therotary shaft while simultaneously rotating about the planetary rolleraxes, respectively; wherein a helical rib is formed on the radiallyinner surface of the outer ring member; and wherein a helical groove isformed in a radially outer surface of each of the planetary rollers,said helical rib being in engagement with the helical grooves of therespective planetary rollers, said helical grooves being equal in pitchto said helical rib, and different in lead angle from said helical rib,so as to cause relative axial movement between said outer ring memberand said planetary rollers when said planetary rollers rotate about saidrotary shaft while simultaneously rotating about the planetary rolleraxes; wherein first thrust bearings are respectively disposed betweenthe planetary rollers and the carrier to support the respectiveplanetary rollers for rotation about the planetary roller axes, and asecond thrust bearing is provided to support the carrier for rotationabout the axis of the rotary shaft; and wherein said helical groovesextend in the same helical direction as said helical rib but at adifferent lead angle than said helical rib.
 2. The electric brakeassembly of claim 1, wherein the helical rib constitutes one of aplurality of helical ribs.
 3. The electric brake assembly of claim 2,wherein the helical groove in each planetary roller constitutes one of aplurality of helical grooves in each planetary roller.
 4. The electricbrake assembly of claim 1, wherein the helical groove in each planetaryroller constitutes one of a plurality of helical grooves in eachplanetary roller.
 5. The electric brake assembly of claim 1, whereinsaid helical rib comprises a rib-forming member received in and fixed toa helical groove formed in the radially inner surface of the outer ringmember.
 6. The electric brake assembly of claim 2, wherein each of saidhelical ribs comprises a rib-forming member received in and fixed to ahelical groove formed in the radially inner surface of the outer ringmember.
 7. The electric brake assembly of claim 5, wherein said electriclinear-motion actuator further comprises means for restricting endsurfaces of said rib-forming member, which is fixed to the helicalgroove.
 8. The electric brake assembly of claim 6, wherein said electriclinear-motion actuator further comprises means for restricting endsurfaces of said rib-forming members, which are fixed to the helicalgrooves.
 9. The electric brake assembly of claim 1, wherein saidelectric linear-motion actuator further comprises means for allowing therotary shaft of the electric motor to be manually rotated.
 10. Theelectric brake assembly of claim 2, wherein said electric linear-motionactuator further comprises means for allowing the rotary shaft of theelectric motor to be manually rotated.
 11. The electric brake assemblyof claim 5, wherein said electric linear-motion actuator furthercomprises means for allowing the rotary shaft of the electric motor tobe manually rotated.
 12. The electric brake assembly of claim 6, whereinsaid electric linear-motion actuator further comprises means forallowing the rotary shaft of the electric motor to be manually rotated.13. The electric brake assembly of claim 7, wherein said electriclinear-motion actuator further comprises means for allowing the rotaryshaft of the electric motor to be manually rotated.
 14. The electricbrake assembly of claim 8, wherein said electric linear-motion actuatorfurther comprises means for allowing the rotary shaft of the electricmotor to be manually rotated.